The present invention relates to subsurface waste water disposal, in particular to processing of waste water in a leach field.
In a waste water treatment system of the type commonly used for domestic dwellings and other limited volume waste water sources, waste water flows first via a sewer line into a primary treatment vessel, such as a septic tank, where it is acted upon by microorganisms in an anaerobic environment. The septic tank effluent, or waste water, then flows through a distribution box and piping to a leach field, for aerobic processing.
There are a variety of types of leach field constructions. Generally, they are characterized by some sort of excavation in the soil that may be a void or may be filled with gravel or crushed stone. In one common construction, perforated distribution pipes run along trenches filled with stone. In another, hollow concrete or plastic chambers or structures are used. Generally, the devices buried in the soil which convey waste water are referred to here as conduits. The conduits receive waste water, and temporarily store it until it infiltrates or disperses into the soil surrounding the conduit.
While seemingly simple in essential construction, the functioning or failing of a leach field, and its restoration, are not so simple. Ian properly functioning leach field biochemical activity makes the waste water environmentally benign. The region of the soil where the preponderance of such biochemical activity takes place is referred to as the influence zone. The influence zone typically extends outwardly from the sides and bottom of the conduit. Its nominal outer bound may vary as a function of many variables, including time, some of which are mentioned here.
A leach field should have sufficient capacity to receive and properly process the anticipated flow of waste water. The steady state capacity of a leach field for waste water flow, sometimes referred to as the percolation rate, is a function of various factors including the nature of the interface between the conduit and the soil and the nature of the surrounding soil, as such natures may have been affected by use of the leach field. Since waste water contains organic substances which are supposed to be oxidized or otherwise acted on by bacteria, conditions in the leach field may be such that, over time, there is an accumulation of such substances within the soil. Thus, the flow resistance or infiltration rate of a leach field will not simply be a function of the permeability, mechanics and hydraulics of the original soil.
In a properly functioning system, according to conventional thinking, the soil surrounding the conduits of a leach field ideally remains predominately unsaturated and aerobic, thus enabling oxidation and destruction of pathogenic bacteria. There can be local anaerobic regions. Nitrogen, discharged in human waste, passes through the influence zone predominantly as ammonium (NH4+), to be nitrified, or converted to nitrate (NO3) form. Foreign constituents in the waste water may also sorb and or react with soil constituents. As the waste water is treated in the influence zone, it moves through the soil, typically toward the natural water table in the earth.
In a properly designed, used and maintained leach field, once biochemical equilibrium is reached, the capacity of the leach field typically remains stable insofar as waste water treatment capacity. However, too frequently, a system will demonstrate insufficient infiltration capacity with the passage of time, and will consider to be failed. The failure might be due to inadequate original design, or to over-use. In both cases, typically there is degradation with time of the capacity of the leach field to properly treat waste water, compared to when it was first installed. Failure of the leach field can be manifested by continuous or intermittent surfacing of inadequately treated waste water to the surface or elsewhere, by inadequate purification of the waste water in the field, or by refusal of the system to accept the ordinary waste water input. And, even if a system has not failed, it is desirable to guard against failure by having the greatest economically feasible margin of safety against failure.
In certain technologically related patent applications discussed further below, different techniques and devices are disclosed for improving, maintaining or correcting the performance of leach fields. There is a need for controlling those different processes and devices, so that the proper result is obtained in an efficient and economic manner, and interdependencies are dealt with. The present invention addresses that need.
An object of the invention is to improve the performance of a leach field of a waste water system. A further object is to correct poor performance in a leach field, particularly one which at or near failure.
In accord with the invention, a leach field is treated by being subjected to controlled amounts of input, or manipulated variable, according to measurements of selected parameters in the field. In the leach field which comprises the conduit, influence zone and other soil, one or more parameter is measured, where the parameter is selected from the group comprising (a) composition of the gas within the soil (b) moisture content within the influence zone; (c) water level in the conduit; (d) gas pressure in the conduit or the soil; (e) temperature in the influence zone. The measured parameter is compared to a desired reference point for the parameter. And, then a manipulated variable is controlled by activation or change, to thereby cause the measured parameter to change in the direction of the desired reference point, where the manipulated variable is selected from the group comprising (a) flowing air or other active gas through the influence zone; (b) heating the influence zone; (c) heating air or other active gas which is flowing through the influence zone; (d) removing waste water from the conduit, including removing waste water in sufficient quantity to cause water to flow from the influence zone into the conduit; (e) lessening that amount of waste water flowing into the conduit.
In further accord with the invention, the manipulated variable being controlled is qualitatively different from the parameter being measured. For example, when the temperature is controlled by raising or lowering the pressure of air supplied to the conduit, the parameter and manipulated variable are qualitatively different.